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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

NICKEL-PLATING

293

2.  Test for Small Articles.—After being freed from grease, the article
or part of it or a number of small pieces, are treated with 8-10 drops of a
mixture of 9 vols. of cone, sulphuric acid with I vol. of cone, nitric acid—a
mixture which readily dissolves the superficial silver but attacks the metal
underneath either not at all or but little.    When the attack is over, the
acid is decanted into a test-tube, mixed with 2-3 c.c. of water, filtered if
necessary and divided into two portions.   To one of these are added 1-2
drops of dilute hydrochloric acid, which are allowed to flow gently down
the wall of the tube and form a layer on the surface of the sulphuric acid
solution.    If silver is present, a more or less distinct milkiness is observed
—best by comparison with the other portion—at the surface.

3.  Test for Large Articles.—In general the technical test (see above)
is applicable in this case.    If, however, the surface or form of the object
renders this difficult the surface or a few scrapings may be treated with
nitric acid diluted with an equal volume of water, care being taken to stop
the action as soon as the superficial silver coloration disappears.    The
nitric acid solution is then decanted into a dish and evaporated to dryness
with a drop of dilute hydrochloric acid on a water-bath.    The residue is
taken up in hot water, acidified with nitric acid and filtered through a small,
very compact filter, which is repeatedly washed with hot water.    A small
quantity of hot, dilute ammonia is then passed a number of times through
the filter and the ammoniacal solution divided into two parts, one of which
is made faintly acid with nitric acid.    In presence of silver a slight precipi-
tate or a more or less marked milkiness is observed.

NICKEL-PLATING

1.  Technical Test.—The surface of the article is treated with a drop
of cone, hydrochloric acid, a crystal of methylamine hydrochloride being
placed close by and heat applied.    In presence of nickel, the place attacked
by the acid exhibits a blue spot which disappears on cooling.

2.  Dimethylglyoxime   Test   (highly  sensitive).—After  being  freed
from grease, the surface of the object is moistened with one or two drops
of nitric acid diluted with an equal volume of water, the acid being sub-
sequently washed into a test-tube, rendered alkaline with ammonia, heated
to boiling, filtered if necessary and treated with two or three drops of i%
alcoholic dimethylglyoxime solution.    In presence of nickel, a red precipitate
or at least a pink coloration is formed.

If this test is applied to an object of copper or brass, a little of the latter
in the solution would give a brown coloration and thus mask the nickel
reaction. In such case, after addition of the glyoxime and gentle heating,
the liquid is filtered ; if nickel is present, a slight red precipitate will remain
on the filter. A better method to follow under these circumstances con-
sists in acidifying the ammoniacal solution with hydrochloric acid, pre-
cipitating the copper by means of hydrogen sulphide, filtering, eliminating
the excess of hydrogen sulphide, rendering slightly ammoniacal, filtering
if necessary, and then adding the alcoholic dimethylglyoxime.o lots of acid, rich in silver nitrate,